Spread spectrum communications techniques have found wide application in wireless communications networks. The main characteristic of these techniques is that the channel is subdivided into data streams via orthogonal codes. In the ideal situation during signal reception the desired signal has perfect correlation with itself and zero correlation with all other signals, including time delayed versions of itself. After passing through a de-spreader, the signal would be recovered having been disturbed only by random noise:xk=ask+n.
In real environments, however, the signals are subject to channel distortion and timing imperfections due to multiple path propagation causing intermixing, which in turn decreases the orthogonality of the signals. The de-spread signal ends up with mixed components of the other signals as well as the desired one:
      x    k    =                    ∑                  j          =          1                n            ⁢                        a          kj                ⁢                  s          j                      +    n  where the term akksk is the desired signal and the other terms are interference:xk=akksk+n1+nwhere n1 is the noise due to the interferers.
Currently, the de-spread signal is processed to determine the symbols with the extraneous terms acting as additional noise. This degrades the result leading to a potential undesirable error rate. High error rates require the use of techniques to provide an adequate integrity to the data stream.
One approach is to increase the use of error correcting codes, which decreases the effective data rate of the link. Another approach is to decrease the symbol rate, which also decreases the effective data rate of the link. Yet another approach is to use negative acknowledgment when the errors exceed the correction capability of the coding, which again decreases the data rate of the link. Other approaches exploit multipath either via MIMO or diversity techniques, which require more complicated circuitry and processing at both the transmitter and the receiver.
The above approaches thus tend to either decrease the data rate of the link, or require more complicated implementations at both the transmitter and receiver. Moreover, requiring implementation at both the transmitter and receiver usually requires standardization, and this leads to incompatibilities with existing radio access networks.